One of the critical equipment functions in the well-known process of pressure injection molding of plastic parts is the control of the temperature of the flowable heated resins that are forced under pressure into mold cavities. As is well known in the art the flowable or molten resins are directed into the mold plates by injection nozzles that pressure feed the resin through the runners that feed into multiple mold cavities. The injection nozzles are required to be associated with a supply of heat in order to assure that flowable properties of the resin melt are at optimum conditions. Maintaining proper temperatures of the resin melt is one aspect of the process that determines the quality of the final molded product or component. Numerous techniques are employed to apply heat to the injection nozzle but the most notable is the coiled heating cable that envelops or surrounds the nozzle and conducts the heat to the resin as it courses or flows through the runner. The coiled heating cables are known to have a defined life expectancy which will depend in part on the manufacture but also is a function of how they are used in the environment of the injection molding process.
It will be appreciated that any improvement in the manner in which the heating coils are utilized in the environment of injection molding apparatus that prolongs the life expectancy of the elements contributes to the efficiency of the apparatus. Efficiency is achieved by less frequent shutdowns to replace burned-out heaters. Further, it has been found that optimal heat utilization, that is directing heat where it is needed, tends to prolong the heater life. The quality of the components produced by injection molding benefits from optimal temperature control because it avoids burning or overheating the resin by excessive heat. In the circumstance that demands are made on the heat system to output unneeded energy to portions of the resin, it tends to overheat the coils. Experienced technicians in this field also understand that overheating is deleterious to the resin as well. An overheated resin will undoubtedly result in a defective molded part.
Optimization of the amount of heat applied to the stream of resin melt going through the injection nozzle requires that it be patterned in terms of providing greater or lesser amounts of heat at certain zones along its path from the manifold to the cavity. Better heater designs create zones that apply different levels of heat transfer from the heating elements. For example, a non-linear heat profile can require varying the energy input anywhere from 20 percent to 40 percent between zones.
The conductivity of the heat energy generated by the heating elements extends axially along the injection nozzle and will encounter different masses of material through which the heat must be conducted before it reaches the molten resins. The point at which the molten resin enters the runner is contained in a rather large mass of metal, the central portion of the injection nozzle in the second zone requires lesser amounts of heat input attributable to the injection molding process itself and the tip or the last heat zone of the injection nozzle requires yet a different heat level requiring larger conductivity because of the thickness of the surrounding apparatus and readying the resin melt for entry into the cavity. Maintaining the desired heat input to the various zones, unless properly profiled, could result in much more heat delivered to the central zone posing a hazard both to the heating coil and to the resin itself. Such differential demand from heat energy, unless it is controlled, causes the heating element adjacent to the central portion to overheat for the reason that the energy is not being consumed. Reducing the heat input so that the central zone does not overheat reduces the heat to the entire system. Too low a heat level may reduce the desired temperature of the resin that is entering the cavity as it leaves the injection nozzle.
As the life of a heating element runs its course, replacement necessitates shutting down the injection molding apparatus; taking it apart to get at the heating element. The less complicated the heating assembly the more easily it can be disassembled and quickly replaced. This represents a significant economic advantage.
There have been attempts in this art at controlling the heat profile of the heating assembly for injection nozzles but they are not without disadvantages. Worthy of comment is the disclosure in U.S. Pat. No. 4,892,474 which employs a pair of flat-faced copper plates which are formed with internally integrated heating elements. These heating plates are bolted to opposite sides of the injection nozzle. These plates are equipped with channels that form insulating air spaces that only partially reach the surfaces of the injection nozzle. This known heating assembly has the disadvantage of providing only partial air insulating areas at the surface of the injection nozzle so the heat profile is discontinuous at best as is the heat which is applied to the injection nozzle where there is no air gap. It will be understood from this prior art teaching that portions of the injection nozzle are devoid of heat input or insulating air gaps.
Another problem is the fact that the heating elements are buried within the plates so that it behaves as a solid heating element on select portions of the injection nozzle leaving large areas of the surface of the nozzle unheated. Replacement of such heating plates is quite costly requiring discarding the entire heating unit and fully replacing it.
It has been found that proper control of the amount of heat energy applied to the resin melt is critical with respect to the quality of the component that is molded and the prolongation of the life of the heating elements. The ease and simplicity and less frequent replacement of numerous spent heating elements during the course of the year significantly affects the productivity and the economics of the injection molding apparatus.